The 'beforeExit' event is emitted when Node.js empties its event loop and has
no additional work to schedule. Normally, the Node.js process will exit when
there is no work scheduled, but a listener registered on the 'beforeExit'
event can make asynchronous calls, and thereby cause the Node.js process to
continue.

The listener callback function is invoked with the value of
process.exitCode passed as the only argument.

The 'beforeExit' event is not emitted for conditions causing explicit
termination, such as calling process.exit() or uncaught exceptions.

The 'beforeExit' should not be used as an alternative to the 'exit' event
unless the intention is to schedule additional work.

Listener functions must only perform synchronous operations. The Node.js
process will exit immediately after calling the 'exit' event listeners
causing any additional work still queued in the event loop to be abandoned.
In the following example, for instance, the timeout will never occur:

If the Node.js process is spawned with an IPC channel (see the Child Process
and Cluster documentation), the 'message' event is emitted whenever a
message sent by a parent process using childprocess.send() is received by
the child process.

The 'rejectionHandled' event is emitted whenever a Promise has been rejected
and an error handler was attached to it (using promise.catch(), for
example) later than one turn of the Node.js event loop.

The listener callback is invoked with a reference to the rejected Promise as
the only argument.

The Promise object would have previously been emitted in an
'unhandledRejection' event, but during the course of processing gained a
rejection handler.

There is no notion of a top level for a Promise chain at which rejections can
always be handled. Being inherently asynchronous in nature, a Promise
rejection can be handled at a future point in time — possibly much later than
the event loop turn it takes for the 'unhandledRejection' event to be emitted.

Another way of stating this is that, unlike in synchronous code where there is
an ever-growing list of unhandled exceptions, with Promises there can be a
growing-and-shrinking list of unhandled rejections.

In synchronous code, the 'uncaughtException' event is emitted when the list of
unhandled exceptions grows.

In asynchronous code, the 'unhandledRejection' event is emitted when the list
of unhandled rejections grows, and the 'rejectionHandled' event is emitted
when the list of unhandled rejections shrinks.

In this example, the unhandledRejectionsMap will grow and shrink over time,
reflecting rejections that start unhandled and then become handled. It is
possible to record such errors in an error log, either periodically (which is
likely best for long-running application) or upon process exit (which is likely
most convenient for scripts).

The 'uncaughtException' event is emitted when an uncaught JavaScript
exception bubbles all the way back to the event loop. By default, Node.js
handles such exceptions by printing the stack trace to stderr and exiting.
Adding a handler for the 'uncaughtException' event overrides this default
behavior.

The listener function is called with the Error object passed as the only
argument.

Note that 'uncaughtException' is a crude mechanism for exception handling
intended to be used only as a last resort. The event should not be used as
an equivalent to On Error Resume Next. Unhandled exceptions inherently mean
that an application is in an undefined state. Attempting to resume application
code without properly recovering from the exception can cause additional
unforeseen and unpredictable issues.

Exceptions thrown from within the event handler will not be caught. Instead the
process will exit with a non-zero exit code and the stack trace will be printed.
This is to avoid infinite recursion.

Attempting to resume normally after an uncaught exception can be similar to
pulling out of the power cord when upgrading a computer -- nine out of ten
times nothing happens - but the 10th time, the system becomes corrupted.

The correct use of 'uncaughtException' is to perform synchronous cleanup
of allocated resources (e.g. file descriptors, handles, etc) before shutting
down the process. It is not safe to resume normal operation after
'uncaughtException'.

To restart a crashed application in a more reliable way, whether uncaughtException
is emitted or not, an external monitor should be employed in a separate process
to detect application failures and recover or restart as needed.

Unhandled Promise rejections have been will now emit a process warning.

v1.4.1

Added in: v1.4.1

The 'unhandledRejection' event is emitted whenever a Promise is rejected and
no error handler is attached to the promise within a turn of the event loop.
When programming with Promises, exceptions are encapsulated as "rejected
promises". Rejections can be caught and handled using promise.catch() and
are propagated through a Promise chain. The 'unhandledRejection' event is
useful for detecting and keeping track of promises that were rejected whose
rejections have not yet been handled.

The listener function is called with the following arguments:

reason<Error> | <any> The object with which the promise was rejected
(typically an Error object).

The following will also trigger the 'unhandledRejection' event to be
emitted:

function SomeResource() {
// Initially set the loaded status to a rejected promise
this.loaded = Promise.reject(new Error('Resource not yet loaded!'));
}
const resource = new SomeResource();
// no .catch or .then on resource.loaded for at least a turn

In this example case, it is possible to track the rejection as a developer error
as would typically be the case for other 'unhandledRejection' events. To
address such failures, a non-operational
.catch(() => { }) handler may be attached to
resource.loaded, which would prevent the 'unhandledRejection' event from
being emitted. Alternatively, the 'rejectionHandled' event may be used.

A process warning is similar to an error in that it describes exceptional
conditions that are being brought to the user's attention. However, warnings
are not part of the normal Node.js and JavaScript error handling flow.
Node.js can emit warnings whenever it detects bad coding practices that could
lead to sub-optimal application performance, bugs or security vulnerabilities.

The listener function is called with a single warning argument whose value is
an Error object. There are three key properties that describe the warning:

By default, Node.js will print process warnings to stderr. The --no-warnings
command-line option can be used to suppress the default console output but the
'warning' event will still be emitted by the process object.

The following example illustrates the warning that is printed to stderr when
too many listeners have been added to an event

Note: An easy way to send the SIGINT signal is with <Ctrl>-C in most
terminal programs.

It is important to take note of the following:

SIGUSR1 is reserved by Node.js to start the debugger. It's possible to
install a listener but doing so will not stop the debugger from starting.

SIGTERM and SIGINT have default handlers on non-Windows platforms that
resets the terminal mode before exiting with code 128 + signal number. If
one of these signals has a listener installed, its default behavior will be
removed (Node.js will no longer exit).

SIGPIPE is ignored by default. It can have a listener installed.

SIGHUP is generated on Windows when the console window is closed, and on
other platforms under various similar conditions, see signal(7). It can have a
listener installed, however Node.js will be unconditionally terminated by
Windows about 10 seconds later. On non-Windows platforms, the default
behavior of SIGHUP is to terminate Node.js, but once a listener has been
installed its default behavior will be removed.

SIGTERM is not supported on Windows, it can be listened on.

SIGINT from the terminal is supported on all platforms, and can usually be
generated with CTRL+C (though this may be configurable). It is not generated
when terminal raw mode is enabled.

SIGBREAK is delivered on Windows when <Ctrl>+<Break> is pressed, on
non-Windows platforms it can be listened on, but there is no way to send or
generate it.

SIGWINCH is delivered when the console has been resized. On Windows, this
will only happen on write to the console when the cursor is being moved, or
when a readable tty is used in raw mode.

SIGKILL cannot have a listener installed, it will unconditionally terminate
Node.js on all platforms.

SIGSTOP cannot have a listener installed.

SIGBUS, SIGFPE, SIGSEGV and SIGILL, when not raised artificially
using kill(2), inherently leave the process in a state from which it is not
safe to attempt to call JS listeners. Doing so might lead to the process
hanging in an endless loop, since listeners attached using process.on() are
called asynchronously and therefore unable to correct the underlying problem.

Note: Windows does not support sending signals, but Node.js offers some
emulation with process.kill(), and ChildProcess.kill(). Sending
signal 0 can be used to test for the existence of a process. Sending SIGINT,
SIGTERM, and SIGKILL cause the unconditional termination of the target
process.

The process.argv property returns an array containing the command line
arguments passed when the Node.js process was launched. The first element will
be process.execPath. See process.argv0 if access to the original value of
argv[0] is needed. The second element will be the path to the JavaScript
file being executed. The remaining elements will be any additional command line
arguments.

If the Node.js process was spawned with an IPC channel (see the
Child Process documentation), the process.channel
property is a reference to the IPC channel. If no IPC channel exists, this
property is undefined.

The process.config property returns an Object containing the JavaScript
representation of the configure options used to compile the current Node.js
executable. This is the same as the config.gypi file that was produced when
running the ./configure script.

If the Node.js process is spawned with an IPC channel (see the Child Process
and Cluster documentation), the process.connected property will return
true so long as the IPC channel is connected and will return false after
process.disconnect() is called.

Once process.connected is false, it is no longer possible to send messages
over the IPC channel using process.send().

The process.cpuUsage() method returns the user and system CPU time usage of
the current process, in an object with properties user and system, whose
values are microsecond values (millionth of a second). These values measure time
spent in user and system code respectively, and may end up being greater than
actual elapsed time if multiple CPU cores are performing work for this process.

The result of a previous call to process.cpuUsage() can be passed as the
argument to the function, to get a diff reading.

If the Node.js process is spawned with an IPC channel (see the Child Process
and Cluster documentation), the process.disconnect() method will close the
IPC channel to the parent process, allowing the child process to exit gracefully
once there are no other connections keeping it alive.

The process.execArgv property returns the set of Node.js-specific command-line
options passed when the Node.js process was launched. These options do not
appear in the array returned by the process.argv property, and do not
include the Node.js executable, the name of the script, or any options following
the script name. These options are useful in order to spawn child processes with
the same execution environment as the parent.

The process.exit() method instructs Node.js to terminate the process
synchronously with an exit status of code. If code is omitted, exit uses
either the 'success' code 0 or the value of process.exitCode if it has been
set. Node.js will not terminate until all the 'exit' event listeners are
called.

To exit with a 'failure' code:

process.exit(1);

The shell that executed Node.js should see the exit code as 1.

It is important to note that calling process.exit() will force the process to
exit as quickly as possible even if there are still asynchronous operations
pending that have not yet completed fully, including I/O operations to
process.stdout and process.stderr.

In most situations, it is not actually necessary to call process.exit()
explicitly. The Node.js process will exit on its own if there is no additional
work pending in the event loop. The process.exitCode property can be set to
tell the process which exit code to use when the process exits gracefully.

For instance, the following example illustrates a misuse of the
process.exit() method that could lead to data printed to stdout being
truncated and lost:

// This is an example of what *not* to do:
if (someConditionNotMet()) {
printUsageToStdout();
process.exit(1);
}

The reason this is problematic is because writes to process.stdout in Node.js
are sometimes asynchronous and may occur over multiple ticks of the Node.js
event loop. Calling process.exit(), however, forces the process to exit
before those additional writes to stdout can be performed.

Rather than calling process.exit() directly, the code should set the
process.exitCode and allow the process to exit naturally by avoiding
scheduling any additional work for the event loop:

// How to properly set the exit code while letting
// the process exit gracefully.
if (someConditionNotMet()) {
printUsageToStdout();
process.exitCode = 1;
}

If it is necessary to terminate the Node.js process due to an error condition,
throwing an uncaught error and allowing the process to terminate accordingly
is safer than calling process.exit().

The process.hrtime() method returns the current high-resolution real time
in a [seconds, nanoseconds] tuple Array, where nanoseconds is the
remaining part of the real time that can't be represented in second precision.

time is an optional parameter that must be the result of a previous
process.hrtime() call to diff with the current time. If the parameter
passed in is not a tuple Array, a TypeError will be thrown. Passing in a
user-defined array instead of the result of a previous call to
process.hrtime() will lead to undefined behavior.

These times are relative to an arbitrary time in the
past, and not related to the time of day and therefore not subject to clock
drift. The primary use is for measuring performance between intervals:

The process.initgroups() method reads the /etc/group file and initializes
the group access list, using all groups of which the user is a member. This is
a privileged operation that requires that the Node.js process either have root
access or the CAP_SETGID capability.

signal<string> | <number> The signal to send, either as a string or number.
Defaults to 'SIGTERM'.

The process.kill() method sends the signal to the process identified by
pid.

Signal names are strings such as 'SIGINT' or 'SIGHUP'. See Signal Events
and kill(2) for more information.

This method will throw an error if the target pid does not exist. As a special
case, a signal of 0 can be used to test for the existence of a process.
Windows platforms will throw an error if the pid is used to kill a process
group.

Note: Even though the name of this function is process.kill(), it is
really just a signal sender, like the kill system call. The signal sent may
do something other than kill the target process.

The process.mainModule property provides an alternative way of retrieving
require.main. The difference is that if the main module changes at
runtime, require.main may still refer to the original main module in
modules that were required before the change occurred. Generally, it's
safe to assume that the two refer to the same module.

As with require.main, process.mainModule will be undefined if there
is no entry script.

The process.nextTick() method adds the callback to the "next tick queue".
Once the current turn of the event loop turn runs to completion, all callbacks
currently in the next tick queue will be called.

This is not a simple alias to setTimeout(fn, 0). It is much more
efficient. It runs before any additional I/O events (including
timers) fire in subsequent ticks of the event loop.

Note: The next tick queue is completely drained on each pass of the
event loop before additional I/O is processed. As a result,
recursively setting nextTick callbacks will block any I/O from
happening, just like a while(true); loop.

The process.release property returns an Object containing metadata related to
the current release, including URLs for the source tarball and headers-only
tarball.

process.release contains the following properties:

name<string> A value that will always be 'node' for Node.js. For
legacy io.js releases, this will be 'io.js'.

sourceUrl<string> an absolute URL pointing to a .tar.gz file containing
the source code of the current release.

headersUrl<string> an absolute URL pointing to a .tar.gz file containing
only the source header files for the current release. This file is
significantly smaller than the full source file and can be used for compiling
Node.js native add-ons.

libUrl<string> an absolute URL pointing to a node.lib file matching the
architecture and version of the current release. This file is used for
compiling Node.js native add-ons. This property is only present on Windows
builds of Node.js and will be missing on all other platforms.

lts<string> a string label identifying the LTS label for this release.
If the Node.js release is not an LTS release, this will be undefined.

If Node.js is spawned with an IPC channel, the process.send() method can be
used to send messages to the parent process. Messages will be received as a
'message' event on the parent's ChildProcess object.

If Node.js was not spawned with an IPC channel, process.send() will be
undefined.

The process.setegid() method sets the effective group identity of the process.
(See setegid(2).) The id can be passed as either a numeric ID or a group
name string. If a group name is specified, this method blocks while resolving
the associated a numeric ID.

The process.seteuid() method sets the effective user identity of the process.
(See seteuid(2).) The id can be passed as either a numeric ID or a username
string. If a username is specified, the method blocks while resolving the
associated numeric ID.

The process.setgid() method sets the group identity of the process. (See
setgid(2).) The id can be passed as either a numeric ID or a group name
string. If a group name is specified, this method blocks while resolving the
associated numeric ID.

The process.setuid(id) method sets the user identity of the process. (See
setuid(2).) The id can be passed as either a numeric ID or a username string.
If a username is specified, the method blocks while resolving the associated
numeric ID.

As a Duplex stream, process.stdin can also be used in "old" mode that
is compatible with scripts written for Node.js prior to v0.10.
For more information see Stream compatibility.

Note: In "old" streams mode the stdin stream is paused by default, so one
must call process.stdin.resume() to read from it. Note also that calling
process.stdin.resume() itself would switch stream to "old" mode.

Writes may be synchronous depending on the what the stream is connected to
and whether the system is Windows or Unix:

Files: synchronous on Windows and Linux

TTYs (Terminals): asynchronous on Windows, synchronous on Unix

Pipes (and sockets): synchronous on Windows, asynchronous on Unix

These behaviors are partly for historical reasons, as changing them would
create backwards incompatibility, but they are also expected by some users.

Synchronous writes avoid problems such as output written with console.log() or
console.error() being unexpectedly interleaved, or not written at all if
process.exit() is called before an asynchronous write completes. See
process.exit() for more information.

Warning: Synchronous writes block the event loop until the write has
completed. This can be near instantaneous in the case of output to a file, but
under high system load, pipes that are not being read at the receiving end, or
with slow terminals or file systems, its possible for the event loop to be
blocked often enough and long enough to have severe negative performance
impacts. This may not be a problem when writing to an interactive terminal
session, but consider this particularly careful when doing production logging to
the process output streams.

To check if a stream is connected to a TTY context, check the isTTY
property.

The process.title property returns the current process title (i.e. returns
the current value of ps). Assigning a new value to process.title modifies
the current value of ps.

Note: When a new value is assigned, different platforms will impose
different maximum length restrictions on the title. Usually such restrictions
are quite limited. For instance, on Linux and macOS, process.title is limited
to the size of the binary name plus the length of the command line arguments
because setting the process.title overwrites the argv memory of the
process. Node.js v0.8 allowed for longer process title strings by also
overwriting the environ memory but that was potentially insecure and
confusing in some (rather obscure) cases.

The process.umask() method sets or returns the Node.js process's file mode
creation mask. Child processes inherit the mask from the parent process. Invoked
without an argument, the current mask is returned, otherwise the umask is set to
the argument value and the previous mask is returned.

The process.versions property returns an object listing the version strings of
Node.js and its dependencies. process.versions.modules indicates the current
ABI version, which is increased whenever a C++ API changes. Node.js will refuse
to load modules that were compiled against a different module ABI version.

Node.js will normally exit with a 0 status code when no more async
operations are pending. The following status codes are used in other
cases:

1Uncaught Fatal Exception - There was an uncaught exception,
and it was not handled by a domain or an 'uncaughtException' event
handler.

2 - Unused (reserved by Bash for builtin misuse)

3Internal JavaScript Parse Error - The JavaScript source code
internal in Node.js's bootstrapping process caused a parse error. This
is extremely rare, and generally can only happen during development
of Node.js itself.

4Internal JavaScript Evaluation Failure - The JavaScript
source code internal in Node.js's bootstrapping process failed to
return a function value when evaluated. This is extremely rare, and
generally can only happen during development of Node.js itself.

5Fatal Error - There was a fatal unrecoverable error in V8.
Typically a message will be printed to stderr with the prefix FATAL
ERROR.

6Non-function Internal Exception Handler - There was an
uncaught exception, but the internal fatal exception handler
function was somehow set to a non-function, and could not be called.

7Internal Exception Handler Run-Time Failure - There was an
uncaught exception, and the internal fatal exception handler
function itself threw an error while attempting to handle it. This
can happen, for example, if a 'uncaughtException' or
domain.on('error') handler throws an error.

9 - Invalid Argument - Either an unknown option was specified,
or an option requiring a value was provided without a value.

10Internal JavaScript Run-Time Failure - The JavaScript
source code internal in Node.js's bootstrapping process threw an error
when the bootstrapping function was called. This is extremely rare,
and generally can only happen during development of Node.js itself.

12Invalid Debug Argument - The --inspect and/or --inspect-brk
options were set, but the port number chosen was invalid or unavailable.

>128Signal Exits - If Node.js receives a fatal signal such as
SIGKILL or SIGHUP, then its exit code will be 128 plus the
value of the signal code. This is a standard Unix practice, since
exit codes are defined to be 7-bit integers, and signal exits set
the high-order bit, and then contain the value of the signal code.